Mantle xenoliths in alkaline lavas of the Kerguelen Islands consist of: (1) protogranular, Cr-diopside-bearing harzburgite; (2) poikilitic, Mg-augite-bearing harzburgite and cpx-poor lherzolite; (3) dunite that contains clinopyroxene, spinel phlogopite, and rarely amphibole. Trace element data for rocks and minerals identify distinctive signatures for the different rock types and record upper-mantle processes. The harzburgites reflect an initial partial melting event followed by metasomatism by mafic alkaline to carbonatitic melts. The dunites were first formed by reaction of a harzburgite protolith with tholeiitic to transitional basaltic melts, and subsequently developed metasomatic assemblages of clinopyroxene + phlogopite ± amphibole by reaction with lamprophyric or carbonatitic melts. We measured two-mineral partition coefficients and calculated mineral–melt partition coefficients for 27 trace elements. In most samples, calculated budgets indicate that trace elements reside in the constituent minerals. Clinopyroxene is the major host for REE, Sr, Y, Zr and Th; spinel is important for V and Ti; orthopyroxene for Ti, Zr, HREE, Y, Sc and V; and olivine for Ni, Co and Sc.

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A basanite dyke in the Kerguelen Archipelago contains abundant composite mantle xenoliths consisting of spinel-bearing dunites cross-cut by amphibole-rich veins. Two types of veins (thick and thin) have been distinguished: the thick veins represent almost complete crystallization products of highly alkaline melts similar to the host basanites, whereas thin veins are precipitates from fractionates of the parental melts to the thick veins. These fractionated fluids are enriched in H₂O relative to the parental melts. The amphiboles in the thin veins are lower in Ti and higher in Nb, Ta, Zr and Hf than amphiboles in the thick veins. This fractionation of high field strength elements (HFSE) is consistent with a combination of the changing composition of the fractionated fluids and the change in intrinsic amphibole–fluid partition coefficients for HFSE in fluids with higher aH₂O and lower aTiO₂. The trace element content of amphiboles disseminated in dunitic wall-rocks is closely related to the composition of adjacent veins and thus these amphiboles are precipitates from fluids percolating into the dunite from the veins. Disseminated amphibole reflects the composition of the percolating melt, which is similar to that of the associated veins.